Vol. 25: 83–96, 2016 AQUATIC BIOLOGY Published August 24 doi: 10.3354/ab00663 Aquat Biol OPENPEN ACCESSCCESS Thermal adaptations of embryos of six terrestrial hermit crab species Katsuyuki Hamasaki1,*, Takahiro Matsuda1, Ken Takano1, Mio Sugizaki1, Yu Murakami1, Shigeki Dan2, Shuichi Kitada1 1Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, Minato, Konan, Tokyo 108-8477, Japan 2Research Centre for Marine Invertebrate, National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Momoshima, Onomichi, Hiroshima 722-0061, Japan ABSTRACT: We evaluated the thermal adaptations of embryos of 6 terrestrial hermit crab species in the family Coenobitidae (genera Birgus and Coenobita): B. latro, C. brevimanus, C. cavipes, C. purpureus, C. rugosus, and C. violascens. Embryos of each species were cultured in vitro at 6 dif- ferent temperatures (18 to 34°C) in artificial seawater to avoid air desiccation; the lower threshold temperatures for embryonic development were estimated using heat summation theory equa- tions. Additionally, partial effective cumulative temperatures (> lower threshold temperature) until hatching were determined for ovigerous females of each species cultured in containers. The relationships between the embryonic growth index values (relative area of the embryonic body vs. total embryo surface) and effective cumulative temperatures were expressed using cubic equa- tions. Lower threshold temperature was estimated to be 12.7 to 14.5°C. The effective cumulative temperature and egg incubation period estimates from the appearance of the embryonic body to hatching were higher in B. latro and C. brevimanus, followed by C. rugosus, C. cavipes, and C. violascens, and lower in C. purpureus, suggesting that C. brevimanus may retain an ancestral thermal adaptation trait for embryos, as in B. latro, which is considered the most ancestral species in the coenobitid phylogeny. Egg size varied among species but did not affect the thermal adap- tations of embryos. The lower effective cumulative temperature and shorter egg incubation period may be advantageous to producing broods during the shorter summer breeding season in C. pur- pureus, which has the northern-most geographical distribution. KEY WORDS: Coconut crab · Land hermit crab · Embryonic development · Lower threshold temperature · Effective cumulative temperature INTRODUCTION during the juvenile stage (Harms 1938, Reese 1968, Kadiri-Jan & Chauvet 1998, Hamasaki et al. 2014b). Terrestrial hermit crabs in the family Coenobitidae Terrestrial hermit crabs live in a potentially desic- diverged from a marine ancestor between 84 and cating environment except during the larval phase. 39 million years ago (Bracken-Grissom et al. 2013). Females extrude their eggs terrestrially, which are They comprise land hermit crabs of the genus subsequently attached to the setae of the pleopods on Coenobita, with 16 species, as well as the coconut the left ventral side of the abdomen (de Wilde 1973, crab Birgus latro (Linnaeus, 1767), which is the only Greenaway 2003, Sato & Yoseda 2008, 2009, Drew species in the genus Birgus (McLaughlin et al. 2010). et al. 2010). The B. latro egg mass is afforded little Land hermit crabs carry gastropod shells; however, physical protection from the environment by the the shell-carrying behavior of B. latro appears only mother’s abdomen and is therefore susceptible to © The authors 2016. Open Access under Creative Commons by *Corresponding author: [email protected] Attribution Licence. Use, distribution and reproduction are un - restricted. Authors and original publication must be credited. Publisher: Inter-Research · www.int-res.com 84 Aquat Biol 25: 83–96, 2016 desiccating conditions. To minimize dehydration of the Temperature is the most important environmental egg mass, ovigerous B. latro females require a high- factor affecting biological processes of ectothermic humidity shelter (Schiller et al. 1991). Additionally, B. organisms, including behavior, physiology, growth, latro females maintain hydration of their eggs by using and survival of all life history stages. Therefore, ecto- branchial water reserves and body fluids while groom- thermic organisms must adapt to the thermal condi- ing the egg mass with their fifth pair of pereiopods tions of their habitats (Stillman & Somero 2000, Hall (Schiller et al. 1991). In contrast, the egg mass of & Thatje 2009), and thus temperature may have acted Coenobita spp. land hermit crabs is well protected as a selective force when C. purpureus diverged in against physical damage and desiccation by the fe- the Ryukyu Archipelago region. We hypothesize that male’s shell (de Wilde 1973). Land hermit crabs carry the thermal adaptations of C. purpureus may differ water within their shells (de Wilde 1973, Greenaway from those of other widely distributed coenobitid 2003), and ovigerous females often moisten their eggs species. Our objective in this study was to test this using shell water but never permanently bathe them hypothesis and discuss thermal adaptations of coeno- (de Wilde 1973). Coenobitid females return to the sea bitid crabs in the context of evolutionary and ecolog- to hatch the embryos (Hartnoll 1988, Schiller et al. ical traits. We investigated thermal adaptations of 1991, Nakasone 2001). Newly hatched larvae develop embryos in 6 coenobitid crabs (B. latro, C. brevimanus, through planktonic zoeal stages to megalopae in the C. cavipes, C. purpureus, C. rugosus, and C. violas- sea (Hamasaki et al. 2015b), similar to marine hermit cens) by estimating the lower threshold temperatures crabs. After settlement, coenobitid megalopae recog- (LTT, °C) at which embryonic development ceases nize and acquire gastropod shells and then migrate and the sum of daily temperatures above LTT, i.e. onto land (Reese 1968, Reese & Kinzie 1968, Harvey effective cumulative temperatures (ECT, degree- 1992, Brodie 1999, Hamasaki et al. 2011, 2015a). days [°D]), required for embryonic development to Coenobitid crabs mainly occur in subtropical and hatching. LTT and ECT have been estimated in tropical coastal regions and have been divided into 2 order to evaluate thermal adaptations of ectothermic groups based on their geographical occurrence pat- organisms including insects (Honˇek 1996, Kiritani terns: (1) widely distributed species in the Indo-West 2012) and decapod crustaceans (Hamasaki 1996, Pacific, and (2) relatively narrower distributed spe- 2002, 2003, Hamasaki et al. 2009). To obtain the bio- cies in particular regions, such as the western Atlantic, logical data for estimating these 2 life history param- West Africa, west coast of North America, Northern eters, we conducted 2 laboratory experiments. In Australia, and the northwestern Pacific (Hartnoll Expt 1, we incubated embryos of each species at dif- 1988, Nakasone 1988, Harvey 1992, McLaughlin et ferent temperatures to estimate the LTT values. In al. 2007, Wang et al. 2007, Hamasaki et al. 2015b). Expt 2, we cultured ovigerous females to hatch their Coenobita purpureus Stimpson, 1858 has a limited embryos and examined the relationships between distribution and mainly occurs in oceanic islands growth of the embryos and ECT values. Additionally, north of 24° N in the Ryukyu Archipelago, Izu Islands, we tested whether or not egg size affected the ther- and the Ogasawara (Bonin) Islands, Japan. This crab mal adaptation traits of the embryos of these coeno- is also found on the Pacific coasts of mainland Japan bitids, because it has been suggested that larger eggs (<35° N) (Hamasaki et al. 2016). Additionally, several show slower developmental rates in closely related widely distributed coenobitids, including Coenobita decapod crustaceans (Wear 1974), and interspecific brevimanus Dana, 1852, C. cavipes Stimpson, 1858, variation in egg size is known for coenobitid species C. rugosus H. Milne-Edwards, 1837, C. violascens (Nakasone 2001). Heller, 1862, and B. latro, commonly occur in the southern oceanic islands, Japan (Nakasone 1988, 2001, Hamasaki et al. 2016). Hamasaki et al. (2016) ex - MATERIALS AND METHODS amined the phylogenetic relationships between C. purpureus and its widely distributed congeners based Culture of test animals on partial 16S mitochondrial rDNA sequences. Their phylogenetic tree demonstrated that C. purpureus We captured ovigerous females of Birgus latro, clustered with C. rugosus. They also hypothesized Coe nobita brevimanus, C. cavipes, C. purpureus, C. that ancestral Coenobita species may have expanded rugosus, and C. violascens during late June to early their distribution into the northwestern Pacific region July of 2005 to 2015 on Hatomajima Island (24° 28’ N, (>24° N) and evolved into C. purpureus in the Ryukyu 123° 49’ E), Ishigakijima Island (24° 23−31’ N, 124° 07− region land masses during the Pliocene. 18’ E), and/or Miyakojima Island (24°43− 50’ N, Hamasaki et al.: Embryos of terrestrial hermit crabs 85 125° 15−21’E), Okinawa Prefecture, Japan, and cul- 2007). We used the simple method developed by tured them until their embryos hatched into larvae Nakata et al. (2004) for artificial incubation of em - that were used in experiments. In 2005, ovigerous bryos of Japanese crayfish Cambaroides japonicus females of B. latro were cultured in a laboratory (29 De Haan, 1841. Crayfish embryos were successfully to 30°C) at Yaeyama Station, Seikai National Fish- reared individually in wells of cell-culture micro - eries Research Institute, Fisheries Research Agency